1. Field of the Invention
The present invention relates generally to heat treatment of precipitation-hardened alloy components and, more particularly, to a method for predicting thermal growth of precipitation-hardened alloy components during heat treatment.
2. Background Art
Precipitation-hardened alloy components are often heat-treated after casting to impart increased mechanical strength to the alloy. The heat treatment process usually comprises a solution treatment stage, a quenching stage, and an aging stage. During the solution treatment stage, the alloy is heated above its solubility limit to homogenize the alloy. The length of time that the alloy is heated above its solubility limit is often dictated by the amount of inhomogeneity in the alloy before heat treatment. During the quenching stage, the alloy is quenched to a relatively low temperature where the homogeneous state of the alloy solution is frozen in. During the aging stage, the precipitation-hardened alloy is aged below the solubility limit, causing precipitates to nucleate, grow and coarsen with aging time.
The yield strength of the precipitation-hardened alloy initially increases during aging, as precipitates act as obstacles for dislocation motion in the material. However, extended aging usually results in the coarsening of precipitates, which decreases the mechanical strength of the precipitation-hardened alloy. An optimum aging time and temperature exists for the precipitation-hardened alloy to achieve its highest strength before the coarsening of precipitates starts decreasing the precipitation-hardened alloy's strength. This heat treatment, i.e., temper, is usually referred to as T6. Determining T6 values for precipitation-hardened alloys usually requires inexact and costly trial and error adjustments to aging time and temperature.
In precipitation-hardened alloys aged for peak strength, a macroscopic, irreversible, dimensional change has been known to occur during extended in-service, high-temperature exposure. This effect is commonly referred to as thermal growth, since the dimensional change is usually positive.
Thermal growth may detrimentally affect the performance of engine parts constructed of precipitation-hardened alloys, such as engine blocks and engine heads. One such deleterious effect is that engine blocks constructed of aluminum precipitation-hardened alloys may fail emission certification tests. This is because fuel can become trapped if there is a height differential between a cylinder bore on an aluminum alloy engine block and a cast iron cylinder liner. Such a differential can be caused by thermal growth in the aluminum alloy engine block during operation of the engine.
As a result of the deleterious effects of thermal growth, a specialized T7 heat-treatment is often devised to overage the alloy beyond its point of peak strength in order to stabilize the precipitation-hardened alloy against thermal growth. The T7 over-aging is typically accomplished by aging either at higher temperatures or longer times than the T6 temper. For example, T6 treatment of an Al 319 aluminum alloy includes aging the alloy for five hours at 190° C. T7 treatment of Al 319 includes aging the alloy for four hours at 260° C.
The use of lightweight, precipitation-hardened alloy components is anticipated to increase dramatically in the following years. As a result, the automotive and other industries will experience an overall increase in costs associated with heat-treating, precipitation-hardened alloy components. Therefore, the optimization of heat treatment of precipitation-hardened alloy components by decreasing aging times and/or aging temperatures would result in significant cost savings.
It would be desirable to provide a method for optimizing heat treatment of precipitation-hardened alloy components by decreasing aging time and/or temperature using thermal growth predictions based on a quantitative model. It would also be desirable to provide a method that predicts the optimum heat treatment aging time and temperature necessary for dimensional stability without the need for inexact and costly trial and error measurements.